Oh, that annoying thing. IIRC, itīs basically an impossible weapon. Martin didn't describe it as a training and exercise thing, but the real thing, just in miniature - which doesn't really work out because you need the weight and mass behind it. That would mean itīs not Longsword but a Broadsword, same mass, shorter blade, something I don't see a 12 yo being able to handle. My guess would rather be an early Arming Sword, size of one of the early roman Short Sword, but rather build for cutting.